Various parts such as metallic parts, plated parts, coated parts, electronic parts, and semiconductor parts suffer adhesion thereto of machine oil and other similar fouling substances while they are in the process of manufacture or assemblage. It has been customary for these defiled parts to be cleaned with fluorinated hydrocarbon (hereinafter referred to briefly as "Freon") type solvents and chlorine type solvents because of their outstanding cleaning power and their nonflammability and consequent freedom from the danger of ignition. As parts subjected to special cleaning, such parts as semiconductor wafers and LCD quality substrates which abhor a water mark intensely and demand ultraprecision cleaning are cited. In the field of cleaning of parts under discussion, the vapor washing (in the drying process) as with Freon 113 has been heretofore adopted as the finishing step of cleaning.
Recently, since the effects which Freon type solvents produce on human bodies and the environment have come to raise a grave issue, however, the desirability of a cleaning method which permits ultraprecision cleaning comparable favorably with that obtainable with a Freon type solvent and avoids exerting any adverse effect on the environment has been finding enthusiastic recognition.
As prospective substitutes for the Freon type solvents, various cleaning aqueous and nonaqueous types have been proposed. For example, the development of Freon type substances having a lower ozone destruction coefficient than the existing Freon type solvents is now under way and the study of feasibility of the aqueous cleaning and the cleaning with an organic solvent containing no chlorine is also under way. The feasibility of using vapor drying with isopropyl alcohol (hereinafter referred to briefly as "IPA"), hot air drying, etc. in the place of vapor drying with Freon type solvents has been under study.
Many of the substitute cleaning agents mentioned above are defective in respect that while they are indeed capable of cleaning given surfaces, the surfaces cleaned therewith are required to be rinsed. The organic type substitute cleaning agents are defective in the sense that they often manifest on human bodies not negligible toxicity. Newly developed Freon type substances are not absolutely incapable of environmental pollution and, therefore, are not regarded as preferred substitutes.
For example, after a given part has been cleaned with an aqueous type cleaning agent and rinsed with water, it is necessary for quality cleaning that the surface of the cleaned part wet with adhering water should be uniformly and quickly dried. By the conventional method for forcibly vaporizing the water as by the hot air drying, however, the prevention of a water mark is very difficult to achieve.
The vapor drying with IPA is indeed capable of yielding comparatively favorable results. When the preceding step is executed by the use of an aqueous cleaning agent and an agent adapted for rinsing, there arises the disadvantage that since water and IPA have close boiling points, the water is removed only with great difficulty from IPA and the water mingling in IPA is liable to form a water mark. This drawback, coupled with the fact that many of the substitute cleaning agents mentioned above require the parts cleaned therewith to be rinsed with water, constitutes an obstacle on the way of practical utility of the method of vapor drying with IPA. Further, IPA is defective in respect that since it has a lower flash point 11.7.degree. C. than normal room temperature, it always entails the possibility of catching fire under normal conditions of use. Further, since IPA exhibits high toxicity to human bodies, an increasingly rigid restriction is imposed on its use in the field of cleaning.
In the circumstances, an earnest desire has been expressed for the development of a cleaning method which produces as effective cleaning as the method resorting to the use of a Freon type solvent without entailing any environmental pollution and which particularly obviates the necessity for using a nonaqueous cleaning treatment anywhere throughout the entire process of cleaning. In the field requiring ultraprecision cleaning, the desirability of a cleaning method which imparts to the cleaned part as high resistance to water mark as attainable by the vapor cleaning with a Freon type solvent without entailing any environmental pollution has been finding enthusiastic recognition.
Many of the non-aqueous type solvents which are regarded as promising substitutes for Freon type cleaning agents are inflammable substances. When the part which has been cleaned with such an inflammable solvent is simply subjected to drying by heating or to vapor drying as with the IPA, therefore, the inflammable solvent adhering to the part has the possibility of catching fire. To preclude this danger, the practice of blowing air on the cleaned part thereby expelling the adhering inflammable solvent from the part is now in vogue. This practice, however, is very uneconomical because the inevitable release of the air impregnated heavily with the inflammable solvent into the ambient air threatens air pollution or fire and even aggravates the consumption of the solvent.
In the circumstances, in respects to the use of an inflammable solvent as a cleaning agent, the desirability of a safe and economical cleaning method which precludes the possibility of entailing air pollution or fire and allows effective recovery of the cleaning agent after use has been finding enthusiastic recognition.
This invention, conceived in the urge to cope with the problems encountered by the prior art as described above, has as one object thereof the provision of a cleaning method and a cleaning apparatus which produce as high cleaning effect as the cleaning with a Freon type solvent or the cleaning by vapor drying and diminish the possibility of environmental pollution. Another object of this invention is to provide, in the field necessitating superprecision cleaning, a cleaning method and a cleaning apparatus which allows impartation to the cleaned part as high resistance to water mark as attained by the vapor cleaning with a Freon type solvent and, at the same time, avoid entailing any environmental pollution. Yet another object of this invention is to provide a safe and economical cleaning method which, in the use of the inflammable solvent as a cleaning agent, precludes the danger of air pollution or fire and allows effective recovery of the used cleaning agent.